A polarizing effect can be generated in glasses containing silver, copper or copper-cadmium crystals. These crystals can be precipitated in a boroaluminosilicate glasses having compositions containing suitable amounts of an indicated metal and a halogen other than fluorine.
The polarizing effect is generated in these crystal-containing glasses by stretching the glass and then exposing its surface to a reducing atmosphere, typically a hydrogen containing atmosphere. The glass is placed under stress at a temperature above the glass annealing temperature. This elongates the glass, and thereby elongates and orients the crystals. The shear stress that acts on the particles is proportional to the viscosity of the glass and the draw speed during elongation. The restoring force that opposes the deformation by the shear force is inversely proportional to the particle radius. Hence, the optimum conditions for producing a desired degree of particle elongation and a resulting polarizing effect at a given wavelength involves a complex balance of a number of properties of the glass and the redrawing process. Once the glass has been elongated, the elongated glass article is then exposed to a reducing atmosphere at a temperature above 120° C., but not higher than 25° C. above the annealing point of the glass. This develops a surface layer in which at least a portion of metal halide crystals present in the glass are reduced to elemental silver or copper.
The use of silver halide as a polarizer material capitalizes on two properties of the silver halide that are (1) the liquid particle is very deformable, and (2) it is easier to make larger and controlled particles sizes. The disadvantages of using silver halide are (1) that one cannot make polarizers that operate at wavelengths shorter than red (approximately 650 nm) because of the refractive index of the silver halide and (2) that the process required a hydrogen reduction step. It is possible to stretch metallic silver particles in glass as described by E. H. Land in U.S. Pat. No. 2,319,816 and later by S. D. Stookey and R. J. Araujo in Applied Optics, Vol. 7, No. 5 (1968), pages 777-779. However, the problems encountered are the control of particle size and distribution, especially for visible polarizer application where the aspect ratio of the particle is small, typically 1.5-2 to 1.
The production of polarizing glass, as is described in the patent references provided below, broadly involves the following four steps:                1. Melting a glass batch containing a source of silver, copper or copper-cadmium and a halogen other than fluorine, and forming a glass body or form from a melt;        2. Heat treating the glass body at a temperature above the glass strain point to generate halide crystals having a size in the range of 500-2000 Angstroms (Å);        3. Stressing the halide crystal-containing glass body at a temperature above the glass annealing point to elongate the body and thereby elongate and orient the crystals; and        4. Exposing the elongated body to a reducing atmosphere at a temperature above 250° C. to develop a reduced surface layer on the body that contains metal particles with an aspect ration of at least 2:1.        
Glass polarizers, the material compositions and the methods for making the glasses and articles made from the glasses have been described in numerous United States patents. Products and compositions are described in U.S. Pat. Nos. 6,563,639, 6,466,297, 6,775,062, 5,729,381, 5,627,114, 5,625,427, 5,517,356, 5,430,573, 4,125,404 and 2,319,816, and in U.S. Patent Application Publication No. 2005/0128588. Methods for making polarizing glass compositions and or compositions containing silver, and/or articles made from polarizing or silver-containing glasses have been described in U.S. Pat. Nos. 6,536,236, 6,298,691, 4,479,819, 4,304,584, 4,282,022, 4,125.405, 4,188,214, 4,057,408, 4,017,316, and 3,653,863. Glass articles that are polarizing at infrared wavelengths have been described in U.S. Pat. Nos. 5,430,573, 5,332,819, 5,300,465, 5,281,562, 5,275,979, 5,045,509, 4,792,535, and 4,479,819; and in non-U.S. patents or patent application publications JP 5-208844 and EP 0 719 741. The Japanese patent publication describes a copper-based polarizing glass instead of a silver-based polarizing glass.
While there have been considerable efforts in the art to improve the polarizing glasses used in optical polarizers and the methods used to make them, there is still considerable need for further improvement. In particular, there is a need for efficient visible light optical polarizers having a polarizing layer thickness in the range of 10-40 μm that can be used in equipment where space is at a premium. It is the object of this invention to provide a polarizing glass, made using non-halide containing silver salts, which can be used for polarizing light in the visible light range of approximately 380-700 nm, and a method of making such glass. It is a further object of this invention to produce a visible light polarizing glass by reducing the silver ion in a glass to silver(0) prior to heat treating and stretching the glass.